The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Commonly used temperature sensors include temperature sensing elements such as thermocouples, resistive temperature devices (RTDs), thermistors, diode sensors, and transistor temperature sensors that are packaged within protective sheaths, such as metal tubes. For example, in a temperature sensor utilizing a thermocouple, the thermocouple includes a junction formed by two conductors of dissimilar materials contained within a temperature sensing end of the sheath. The two conductors extend through a protective sheath and are connected to a remote temperature reading device. The thermocouple and conductors are held in place by protective and/or mounting material and, at least partially, isolated from the sheath. However, due to the nature of the protective and/or mounting material, the temperature sensors are often delicate and susceptible to breakage and/or failure.
Various attempts have been made to provide a robust configuration for temperature sensors that are both durable and cost-effective to manufacture. One common temperature sensor is manufactured by hard firing an insulator. The conductor wires are then slid inside the hard fired insulators and the subassembly is then slid into a metal sheath. Insulating powders may also be included within the voids of this assembly. This method produces a temperature sensor that has a hard brittle ceramic insulator that cannot be bent and is susceptible to damage from vibration or impact. Additionally, the process of inserting and firing the insulating power can be time consuming and costly.
Other methods by which temperature sensors are manufactured include a preformed crushable ceramic insulator disposed around the conductors before the sheath is assembled. The conductors are placed inside the preformed crushable insulator and the insulator with conductors are placed into a metal sheath having two open ends with the temperature sensing element being located near one end of the sheath. Thereafter, a portion or all of the sheath is swaged to reduce the outer diameter to form a narrowed portion, thereby compacting the crushable ceramic insulator into a compacted crushed powder around the conductors. As a result, the conductors are held in place by the crushed and compacted ceramic powder with the sensing element remaining in an open un-insulated inner space defined by the narrowed portion. After the sheath is swaged, a ceramic insulating powder is introduced through the open end with the sensing element, is packed around the sensing element, and then that open end of the sheath is closed about the sensing element. However, the ceramic insulating powder can often be loosely distributed around portions of the conductors and the thermocouple and does not provide an effective barrier to moisture for the temperature sensing element.
In a similar manner, the sensing element and conductors can be placed into a metal sheath having one closed end and one open end with the sensing element being located within the closed end. In this case, the sensing element often is not encased and is positioned within a void within the closed end.
Each of these existing manufacturing methods are time consuming and result in a higher cost for the manufactured temperature sensors. Additionally, due to the brittle nature of the fired ceramic powder, temperature sensors manufactured according to existing practice are susceptible to damage and premature failure.